Internal combustion engines typically include a plurality of pistons which are disposed within a plurality of corresponding combustion cylinders. Each of the pistons is pivotally connected to one end of a piston rod, which in turn is pivotally connected at the other end thereof with a common crankshaft. The relative axial displacement of each piston between a top dead center (TDC) position and a bottom dead center (BDC) position is determined by the angular orientation of the crank arm on the crankshaft with which each piston is connected.
A free piston internal combustion engine likewise includes a plurality of pistons which are reciprocally disposed in a plurality of corresponding combustion cylinders. However, the pistons are not interconnected with each other through the use of a crankshaft. Rather, each piston is typically rigidly connected with a plunger rod which is used to provide some type of work output. In a free piston engine with a hydraulic output, the plunger is used to pump hydraulic fluid which can be used for a particular application. Typically, the housing which defines the combustion cylinder also defines a hydraulic cylinder in which the plunger is disposed and an intermediate compression cylinder between the combustion cylinder and the hydraulic cylinder. The combustion cylinder has the largest inside diameter; the compression cylinder has an inside diameter which is smaller than the combustion cylinder; and the hydraulic cylinder has an inside diameter which is still yet smaller than the compression cylinder. A compression head which is attached to and carried by the plunger at a location between the piston head and plunger head has an outside diameter which is just slightly smaller than the inside diameter of the compression cylinder. A high pressure hydraulic accumulator which is fluidly connected with the hydraulic cylinder is pressurized through the reciprocating movement of the plunger during operation of the free piston engine. An additional hydraulic accumulator is selectively interconnected with the area in the compression cylinder to exert a relatively high axial pressure against the compression head and thereby move the piston head toward the TDC position. The TDC position and the BDC position may change from one stroke to the next.
In a free piston engine with a hydraulic power output as described above, the pressure chamber in the hydraulic cylinder which carries the plunger is only connected with the high pressure hydraulic accumulator when the piston head is moving toward the BDC position during a return stroke. During a compression stroke, only a low pressure hydraulic accumulator is connected with the pressure chamber in the hydraulic cylinder which carries the plunger. Since the high pressure fluid in the compression cylinder acts to move the piston head toward the TDC position, and since the cross-sectional area of the plunger head is relatively small and hence does not proportionately significantly add a large amount of additional axial force to the plunger, the high pressure hydraulic accumulator is not connected with the pressure chamber in the hydraulic cylinder during the compression stroke to avoid bleeding off any of the pressure previously built up in the high pressure hydraulic accumulator.